The present invention relates to an environmental control apparatus for controlling a working environment in an exposure apparatus. More particularly, the present invention is concerned with an environmental control apparatus for an exposure apparatus for effecting exposure using exposure light having a wavelength range in which oxygen absorbs the exposure light.
In recent years, it has been desired to produce highly integrated semiconductor circuits. In order to produce such semiconductor circuits, exposure apparatuses have been widely used in which a circuit pattern formed on a reticle as a mask is transferred to a photosensitive substrate. Hence, there have been increasing demands for an exposure apparatus which is capable of forming an image of high resolution. As a technique for producing highly integrated semiconductor circuits, there has been known a technique which utilizes, as exposure light, light having a short wavelength, such as a g-line or i-line light emitted from a mercury lamp, KrF excimer laser light and ArF excimer laser light. Especially, as a technique for producing a DRAM (Dynamic Random Access Memory) of 1G, a technique which utilizes ArF excimer laser light having a wavelength as short as 193 nm has been proposed.
However, when ArF excimer laser light having a wavelength of 193 nm is used as exposure light for producing highly integrated semiconductor circuits, problems arise, such that the amount of exposure light which reaches the photosensitive substrate decreases and, therefore, becomes unstable during exposure. The causes of the above-mentioned problems are as follows:
(1) Light having a short wavelength, such as ArF excimer laser light of 193 nm, is absorbed by oxygen in an atmosphere. That is, ArF excimer laser light has a spectral range which includes a specific wavelength range in which oxygen absorbs the ArF excimer laser light. Therefore, in an exposure apparatus in which ArF excimer laser light is used as exposure light, when exposure is effected in a normal atmosphere comprising air having an oxygen content of about 20%, ArF excimer laser light is absorbed by oxygen during exposure. Therefore, the amount of exposure light which reaches the photosensitive substrate decreases during exposure. PA1 (2) When oxygen absorbs ArF excimer laser light, ozone is produced. Since ozone also absorbs ArF excimer laser light, the amount of exposure light which reaches the photosensitive substrate progressively decreases and therefore becomes unstable during exposure, leading to difficulty in monitoring the amount of exposure light for which highly precise measurements are required. PA1 (1) When an area including a light path is completely sealed, it is difficult to load and unload a mask and a photosensitive substrate in the light path. PA1 (2) When an area including a light path is adapted to be sealable using a gate for loading and unloading a mask and a substrate, which is provided in a carrying means for the mask and substrate, it is difficult to periodically perform maintenance for a substrate stage. PA1 (3) A sealing structure which has a large internal volume and is very complicated is required, due to a large degree of motion of a substrate stage. PA1 (4) It is difficult to form a sealing structure having a sufficient pressure resistance against a difference in pressure between the interior and exterior thereof. PA1 (5) When light is transmitted through a sealing structure having a large internal volume, the temperature distribution in the sealing structure becomes non-uniform, so that it is difficult to control the temperature of the sealing structure.
Accordingly, it is presumed that the above-mentioned problems accompanying a technique which utilizes exposure light having a short wavelength can be obviated by removing oxygen in an area including a light path in an exposure apparatus in a manner such that the area is purged with an inert gas and sealed, or an inert gas is continuously supplied to the area for removing oxygen.
However, when the removal of oxygen in an area including a light path is conducted in such a manner as mentioned above, there are various disadvantages as follows: